The directional solidification process is a version of investment casting in which a cavity resembling the desired finished piece is defined by a ceramic shell mold. The mold is placed on a cooling surface, preheated to a desired temperature in a high temperature environment, filled with a liquid alloy, and withdrawn from the high temperature environment into a lower temperature environment (defined by a vacuum or liquid coolant or cooling by other means) at a specific rate so as to induce solidification of the liquid alloy in a directional fashion, starting at the cooling plate. A casting furnace is known for example from U.S. Pat. No. 3,532,155, furnaces working with gas cooling are known from the U.S. Pat. No. 3,690,367 or the European patent application EP-A1-749,790, and a LMC furnace is described in U.S. Pat. No. 3,763,926.
Directionally solidified articles with a columnar grain structure contain a multitude of grains oriented within a certain controlled and generally narrow range of angles relative to the main direction of stress in the article during service. For example, the direction of centrifugal force in a turbine blade is considered along the main axis of the blade and runs from root to blade tip. The preferred direction of grain growth is always parallel to the direction of heat flow during directional solidification.
Due to the nature of the directional solidification process, this main axis of the component to be solidified is normally oriented vertically so that the grains naturally grow along the main stress axis. The cooling plate is oriented horizontally. This is described by Chandley in U.S. Pat. No. 3,248,764 and by VerSnyder in U.S. Pat. No. 3,260,505. In these disclosures, an open-ended “starter zone” incorporated into the bottom of the ceramic casting mould is placed directly over a chill plate.
When liquid alloy is poured into the ceramic shell mold, it impinges on the chill plate and solidification starts immediately, where many randomly oriented grains begin growing at the chill plate. In a “starter zone” which restricts growth to the direction parallel to the imposed thermal gradient and solidification direction (perpendicular to the advancing solid front—perpendicular also to the cooling plate), the grains growing most closely to the direction of heat flow (in this case, the vertical) will grow the fastest and crowd out those that have larger angles to this preferred direction.
At leaving the exit of the starter section there are typically many grains growing approximately in the same direction. In this sense, starter zones are grain selectors in that they impede the growth of grains of undesirable crystallographic orientations into the article to be manufactured. Typical starter zones consist simply of rectangular or angled blocks connected directly to the article to be solidified with a columnar grained structure. Typically the growth direction is vertical and the chill plate and induced isotherms (and solid front) are horizontal. For columnar grained articles, the starter block is connected directly to an article-defining cavity in the shell mold.
A modification of this is given by US 4,475,582, US 4,548,255 and US 4,180,119 in which a smaller starter block is used, at the top of which there is a helical “pig tail” type grain selector which is more effective than the vertically oriented starter—so much so that only one dendrite with an orientation very closely oriented to the vertical exits at the top of the helical selector and enters the article, thereby imparting the article with a single crystal grain structure. The helical selector, in effect, selects out the single best oriented dendrite or grain from those exiting the first starter which are generally vertically oriented but within a certain distribution of angles to the vertical. A less effective means of selecting a vertically oriented dendrite is with a simple thin and elongated growth section leading to the article-defining cavity, as disclosed by Bridgman in U.S. Pat. No. 1,793,672.
In both of the above cases (columnar grained and single crystal selectors) the selectors produce an array of grains or a single grain with random rotational orientations where the axis of rotation is equivalent to the direction of preferred growth (generally the vertical)—this is referred to as the secondary orientation. In many cases the random secondary orientation is not a problem for the design of a single crystal article (the well known advantages of a defined primary orientation are much more important) and hence many are cast with random secondary orientations. However in some cases there are definite advantages to growing a single crystal article having a defined secondary orientation.
For these cases there exist practices and disclosures for selecting grains with a controlled secondary orientation. These fall into two categories: using seed crystals (also described by Bridgman in U.S. Pat. No. 1,793,672) and using special grain selectors. Seed crystals are generally themselves small cylindrical single crystal castings fit into the bottom of the shell mold, and liquid metal filing the shell mold impinges on this rather than on a chill plate. If the seed crystal is properly used, solidification will start epitaxially at the seed-liquid interface, continuing desirable primary and secondary crystallographic orientations of the seed throughout the article-defining cavity.
These seed crystals are not without problems, being costly to produce and requiring special conditions during casting to produce the desired single grain in the article. To reduce the cost they are made small, but this also makes them difficult to handle and difficult to reuse. They constitute another handling operation for the shell mold and impose a holding chamber on the shell mold design. The cavities designed to contain the seeds must function precisely to avoid liquid metal leaking around the sides of the seeds which would nucleate new, randomly oriented grains. For this reason there are several disclosures for using single crystal selectors in addition to the seed in order to block out the continued growth of the random grains (see US 4,714,101, US 4,475,582). Finally, the small passage way containing the seeds and the passage from the seeds to the article-defining cavity in the shell mold almost completely eliminate any significant heat transfer through the metal to the cooling plate, thus slowing down solidification during the casting process. This increases the furnace time required to cast the part, and hence increases its cost.
Because of the problems associated with seeds there have also been disclosures of specially designed single crystal selectors which, by themselves, produce single grains with the desired primary vertical orientation and also a desired secondary orientation. For example, U.S. Pat. No. 3,580,324 discloses a selector with right angle bends and horizontal growth directions to select a secondary orientation. U.S. Pat. No. 5,062,468 discloses a selector design that produces dendrites which are almost always within +/−30° of the desired secondary orientation by using special horizontal growth sections. Numerous other disclosures are given for various means of selecting the secondary orientation of a single grain.
As discussed, there are sometimes advantages for controlling the secondary orientation of the grain for a single crystal article. For a columnar grained article, the random secondary orientation of the columnar grains with respect to the vertical has always been taken as unavoidable and is evidenced by the current standard practice of casting columnar grained articles with such grains of random secondary orientation. However, if it were to be desired to control the secondary orientation of these grains, seeds and selectors would still be the only possible means of achieving this. The same problems of seeds for single crystal articles would exist for seeds to be used for columnar grained castings—and would be much greater. The seeds, being composed of several individual single crystals and generally being much larger in surface area than seeds for single crystal articles would be very expensive. Fitting into the shell mold would be even more problematic. In fact, once a seed is going to be used, greater advantages in properties of the article and simplicity in seeding are maintained simply by using a single seed for the part. The multiple orientation seed is a contradiction in terms, since if all members in the seed were given the same orientation, a single crystal structure would result with the widely known superior properties over columnar grained articles.